September 30, 2014

History of the Atom

• We know today that all matter is composed of atoms.• Atoms: basic unit of matter. Atoms are the smallest

particle of an element that retains its identity.

How did scientists figure out the structure of the atom? Can you just "look" at it?

September 30, 2014

Atomos = indestructible

(Democritus)

http://home.wlu.edu/~mahonj/Ancient_Philosophers/Atomists.htm

• 460-370 B.C.• He hypothesized that all matter is composed of tiny

indestructible units called atoms.• Atoms and voids• Atoms are solid.• Different shapes = different properties.• Not based on scientific method.

September 30, 2014

John Dalton

• 1766-1844• English chemist and a schoolteacher• Used the scientific method to study Democritus's

ideas> Proposed his Atomic Theory

http://www.biography.com/people/john-dalton-9265201

September 30, 2014

Dalton's Atomic Theory (1808)

1. All matter is made of indestructible particles called atoms.

September 30, 2014

Dalton's Atomic Theory (1808)

2. All atoms of the same element are identical.

September 30, 2014

Dalton's Atomic Theory (1808)

3. Atoms cannot be created or destroyed in chemical reactions: They just rearrange.

4. Law of definite proportions: In a chemical reaction, atoms of one element combine with atoms of other elements to form compounds in whole ratios.

+

September 30, 2014

J. J. Thomson

• 1856-1940• English physicist• Cathode ray tube experiment

http://wps.prenhall.com/wps/media/objects/602/616516/Media_Assets/Chapter02/Text_Images/FG02_03.JPG

September 30, 2014

J. J. Thomson

• Cathode rays are rays emitted when high voltage is applied between two electrodes in an evacuated glass tube.

• Observations:> The cathode ray is deflected by a magnetic field.> The cathode ray is repelled by a negative electric

field.> The cathode ray is the same kind of negative

particle no matter what metal is used.

September 30, 2014

JJ Thomson knew that an atom is neither positively or negatively charged (neutral) and contains electrons.

What model of the atom explains J. J. Thomson's findings? How do you think he conceptualized the atom?

J. J. Thomson

• Conclusions:> Cathode rays are streams of negatively charged

particles. – Discovered first subatomic particle: electrons.

> Determined the charge to mass ratio (e/me)

September 30, 2014

William Thomson (Lord Kelvin)

• Plum pudding model (or chocolate chip ice cream)> Uniform "pudding" of positive charge with

electrons scattered throughout.> Total charge is neutral.

http://en.wikipedia.org/wiki/Plum_pudding_modelhttp://en.wikipedia.org/wiki/Christmas_pudding

September 30, 2014

Ernest Rutherford

• 1871-1937• New Zealand physicist• Gold foil experiment

http://staff.norman.k12.ok.us/~cyohn/index_files/atom1notes.htm

September 30, 2014

Can this happen with the plum pudding model?

What is a better model for the atom?

Ernest Rutherford

• Observations:> Most of the alpha ( α) particles (positively charged

particles) passed straight through the foil.> Some were deflected at slight angles, a few even

backwards.

September 30, 2014

Ernest Rutherford

• Conclusions: > Atom is mostly empty space.> Atom's mass is concentrated at the center

(nucleus)> Nucleus has a positive charge-made of protons

http://myweb.usf.edu/~mhight/goldfoil.html

September 30, 2014

James ChadwickDiscovered neutrons--a particle in the nucleus with no charge (neutral)

http://www.tvguide.com/tvshows/the-adventures-of-jimmy-neutron-boy-genius/photos/195915

September 30, 2014

Robert Milikan

• 1868-1953• Oil drop experiment

> Suspended negatively charged oil droplets between two charged plates.

> Calculated how strong an applied electric field has to be to stop the drop from falling due to gravity.

September 30, 2014

Robert Milikan

• He noticed that the charge was always a multiple of -1.60 x 10-19 Coulombs.

• Conclusions: The charge of an electron is -1.60 x 10-19

Coulombs.

September 30, 2014

We know much more now than Dalton did when he first described his "Atomic theory". Some of the laws still hold true:

• Law of Conservation of Mass-In a chemical reaction, atoms (and therefore mass) are never lost or gained, only rearranged.

• Law of Definite Proportions (constant composition)-in a pure compound, the proportions of elements by mass are always the same.

September 30, 2014

1 Which experiment showed that the atom has a dense, positive region called the nucleus?

A Rutherford's Gold Foil Experiment

B Milikan's Oil Drop Experiment

C JJ Thomson's Cathode Ray Experiment

September 30, 2014

2 Milikan's Oil Drop Experiment allowed the calculation of the charge of the electron

True

False

September 30, 2014

http://kids.britannica.com/comptons/art-156255/In-Niels-Bohrs-model-of-the-atom-electrons-can-circle

Parts of the Atom

September 30, 2014

Parts of the Atom

• Proton: positively charged particle found in the nucleus (charge opposite of electron)> Mass of 1.0073 amu (2000x bigger than electron)> Atomic number: (Z) # of protons in nucleus,

identifies elements• Neutron: neutral particle found in the nucleus

> Mass of 1.0087 amu (about the same as a proton)> Number of neutrons determines isotopes

http://chemistry.tutorcircle.com/inorganic-chemistry/atomic-structure.html

September 30, 2014

Parts of the Atom

• Electron: negatively charged particle that occupies a 3D region around the nucleus called orbitals> Mass of 5.5 x 10-4 amu (1/2000 the mass of a

proton)> Electrons are involved in chemical reactions.

http://chemistry.tutorcircle.com/inorganic-chemistry/atomic-structure.html

September 30, 2014

Parts of the Atom

• The nucleus makes up a very small part of the atom's volume.

• The electrons occupy a 3D regions of space called orbitals that surround the nucleus.

http://kids.britannica.com/comptons/art-156255/In-Niels-Bohrs-model-of-the-atom-electrons-can-circle

September 30, 2014

Parts of the Atom

September 30, 2014

3 Which subatomic particle has a neutral charge and is found in the nucleus?

A Electron

B Proton

C Neutron

September 30, 2014

4 Which subatomic particle is represented by the symbol p+

A Electron

B Proton

C Neutron

September 30, 2014

5 Which subatomic particle has the smallest mass?

A Electron

B Proton

C Neutron

September 30, 2014

Which element has 35 protons?

Which element is Z=12?

Elements

• Atomic number (Z): Number of protons in their nucleus. Defines elements.

http://chemistry.about.com/od/workedchemistryproblems/a/isotopes-nuclear-symbols-1.htm

September 30, 2014

Ions

• Ions are atoms that have gained or lost electrons.• # electrons ≠ # protons

> e- > p+ anion (negative charge) Xn-

> e- < p+ cation (positive charge) Xn+

http://www.gcsescience.com/a4-sodium-ion.htm

September 30, 2014

Example 1:

• Write the symbol for a sodium that has lost 1 electron.

• Write the symbol for a magnesium that has lost 2 electrons.

• Write the symbol for a oxygen that has gained 2 electrons.

• Write the symbol for a fluorine that has gained 1 electron.

September 30, 2014

6 Which symbol represents a chlorine that has gained one electron?

A Cl

B Cl-

C Cl+

September 30, 2014

7 Which element has Z=47?

A Au

B Ag

C Ti

September 30, 2014

Isotopes

• Isotopes: Atoms of the same element with different number of neutrons> Isotopes have the same # of protons, different #

of neutrons• Example: Hydrogen had 3 naturally occurring isotopes.

http://wikis.lawrence.edu/display/CHEM/5.+Isotopes+and+Ions+%28Brittany+Oleson%29

September 30, 2014

http://intro.chem.okstate.edu/1215F99/Lecture/Lec101599.html

Atomic number (Z)

Average atomic mass

Isotopes

• Atomic mass: total mass of an atom.• Atomic mass unit (amu): unit to measure atomic

mass.> 1 amu = 1.66 x 10-24g> 1 amu = 1/12 the mass of a C-12 atom.

• Average atomic mass: weighted average mass of all isotopes of an element that occurs in nature.

massavg = %1m1 + %2m2 + ...

September 30, 2014

Example 2: Silicone

mass number exact mass % abundance

28 27.976927 92.23

29 28.976495 4.67

30 29.973770 3.10

massavg = %1m1 + %2m2 + ...

September 30, 2014

Example 3: Carbon

mass number exact mass % abundance

12 12.000000 98.90

13 13.003355 1.10

massavg = %1m1 + %2m2 + ...

September 30, 2014

Example 4: Copper

Copper has two naturally occuring isotopes. Cu-63 has an atomic mass of 62.9296 amu and an abundance of 69.15%. What is the atomic mass of the second isotope?

massavg = %1m1 + %2m2 + ...

September 30, 2014

Standard Nuclear Notation

• Atomic number (Z) = # of protons• Mass number (A) = # of protons + # of neutrons

> Identifies isotopes

http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/nucnot.html

September 30, 2014

Example 5Write the standard nuclear notation for the following:

• An atom with 8 protons and 9 neutrons.

• An atom with 15 protons and 15 neutrons.

• An atom with 34 protons and 38 neutrons.

September 30, 2014